U.S. Pat. No. 4,621,882 granted Nov. 11, 1986 teaches a type of thermally responsive electrical connector which utilizes a shape memory alloy to effect contact closure. The particular shape memory alloy in this patent operates to close the connector or, in an alternatively suggested embodiment, open the connector responsive to heating and refers to earlier U.S. Pat. Nos. 3,606,592 and 4,018,547 which describe the shape memory phenomenon and further refers to a shape memory alloy Nitinol which has a transition temperature range (TTR) which can be varied depending upon the alloy. TTR values between -60 degrees Fahrenheit and +300 degrees Fahrenheit are known.
U.S. Pat. No. 4,911,643 issued Mar. 27, 1990 is drawn to a high density and high signal integrity connector based upon shape memory alloy positioned within a tube and operable to drive contact pads on a flexible dielectric film to open and close against a printed circuit board and the conductive pads thereon. This patent mentions a high density interconnect as being related to 100 to 250 conductor lines per inch, such density being achieved by the forming of conductive paths and conductive pads on flexible film. The patent further teaches the use of a spring member which is contiguous to the film carrying paths and pads and coextensive therewith and is given an elongated hollow split tube geometry. A heat recoverable member of shape memory alloy is positioned concentrically within the spring member and is operable in its low strength state to be overcome by the spring member but in its high strength state to in essence overcome the spring member and open the connector in effect disconnecting the various pads. This patent mentions a reference to shape memory alloys as "Shape Memory Alloys" by L. Donald Schetky, Scientific America, Volume 281, November 1979, pages 74-82. In that publication and in other cited publications, the point is made that the transition temperatures for shape memory alloys allowing them to go from a high strength state to a low strength state and reverse can be quite varied by varying the alloy and by varying the "training" of the material.
U.S. patent application Ser. No. 07/387,269 filed July 28, 1989, now abandoned, relates to high density electrical connectors utilizing canted coil springs as the force driving mechanism to effect interconnection between circuit paths and pads such as are found on printed circuit boards. It has been found that the canted coil spring, as will be explained hereinafter, provides normal forces quite sufficient to effect stable electrical interconnections between contact pads linearly, along the length of the spring, while at the same time providing discretely resilient points of force generation. Because of this, canted coil springs are uniquely suited to high density interconnections wherein radial tolerance variations along the linear length of contact arrays otherwise limit individual action by conventional springs which are tied closely together or common along such linear length.
With respect to high density connectors of the type being discussed, one of the problems is that if contact springs have enough force to generate sufficient normal contact force, the forces of insertion and withdrawal with respect to high density connectors where there are dozens, if not hundreds, of contact pads make circuit board insertion and extraction difficult with possible damage to the contacts or components carried on such boards. For this reason, a number of connectors have been developed which are termed LIF connectors for low insertion force uses and ZIF for zero insertion force uses. The earlier mentioned patents dealing with shape memory alloy elements purport to provide either of these types of connectors. The last mentioned applications are not intended to provide either of these types of interconnections.
Accordingly, it is an object of the present invention to provide a low force electrical connector for high density electrical interconnection applications which can be operated between two conditions, a low force engagement and a high force engagement. It is a further object to provide a low cost means effective to cause interconnections of contact pads on close centers by individual spring members in the form of a canted coil structure itself made of a shape memory alloy responsive to temperature variations to effect forces for insertion, withdrawal, and mating contact. It is yet a further object to provide a novel interconnection medium capable of providing in one piece large numbers of individual force vectors along the linear length thereof which can be varied in response to selected temperature changes.